1. Field of the Invention
The present invention relates to a cut sheet feeder for use with the image forming apparatus such as a copier or a printer, the feeder feeding the apparatus with cut sheets of recording paper.
2. Description of the Related Art
The image forming apparatus is most often used to record image on a large number of cut sheets of the same size which are fed continuously. But occasionally, it is desired to interrupt the ongoing process of image printing involving many cut sheets so as to insert image printing of a different kind typically on a smaller number of sheets. Some of the conventional cut sheet feeders for use with the image forming apparatus have mechanisms that meet the requirements for the on-demand inserted printing.
Such conventional cut sheet feeders have a plurality of sheet feed mechanisms positioned near the sheet feed table or sheet cassette assembly, each mechanism comprising its dedicated pick-up rollers, guide plates and other related parts. When the operator selects a desired sheet type, the corresponding sheet feed mechanism is activated. The activated mechanism picks up cut sheets one by one from the applicable sheet feed table or from the corresponding sheet cassette and feeds them into a position inside the image forming apparatus.
The conventional cut sheet feeder of the above-mentioned type has as many sheet feed mechanisms as the number of the sheet feed tables or the sheet cassettes that are mounted in advance of feed operation. The cut sheet feeder also has a plurality of sheet transport routes for guiding cut sheets from the multiple storage locations via rollers to a common feed position. These features combine to make the cut sheet feeder bulky, complex, and thus prone to feed-related troubles.
Furnishing the multiple sheet feed tables entails the drawback of a reduced amount of cut sheets that may be placed on each table. Where the image forming apparatus is a high-speed type, the limited quantities of cut sheets stacked on the sheet feed tables make it necessary for the operator frequently to replenish cut sheets. On the other hand, there exist cut sheet feeders having only one sheet feed table that carries a large number of cut sheets. With the signal table structure, the sheet feed table moves vertically to keep feeding the many cut sheets it carries into the sheet feed mechanism. This type of cut sheet feeder has difficultly in allowing the operator to switch cut sheets halfway or to interrupt the ongoing bulk printing to effect on-demand inserted printing on a different type of cut sheets.
With these problems taken into account, the inventor of this invention devised a cut sheet feeder having a single sheet feed mechanism to which any one of a plurality of sheet feed tables carrying cut sheets and moving up and down is positioned as desired. With this cut sheet feeder, arrangements need to be made so that one table will not interfere during its vertical movement with any other table. Arrangements must also be made to ensure that any one table does not carry excess cut sheets that would hamper the vertical movement of any other table. The excess cut sheets stacked on any table can disable the entire sheet feed operation.